Bidirectional multiplexers/demultiplexers, i.e. triplexers and diplexers, serve as optical gateways between a fiber to the home (FTTH) optical network and subscribers' homes. Triplexers and diplexers are extremely compact and low-cost access devices capable of receiving one (diplexer) or two (triplexer) high-speed channels (e.g. 1490 nm for telephone & internet, and 1550 nm for video), while simultaneously transmitting on a third channel (e.g. 1310 for information out). All these signals are multiplexed onto a single optical fiber for simple installation.
Typical triplexer requirements present considerable challenges for conventional optical component designers. Triplexer optical architecture requires that a laser, nominally 1310 nm in wavelength, is coupled to a single-mode fiber for transmitting optical signals from the subscriber's home. In the other direction on that same fiber, light at wavelengths of nominally 1490 nm and 1550 nm from outside the home are captured, demultiplexed and directed to optical detectors. Difficulties arise due to the operational passbands at the various wavelengths, i.e. at the 1310 nm channel, a band of 50 nm to 100 nm is expected, which provides a large margin within which the laser can operate essentially athermally, whereas bands of only 10 nm to 20 nm width are required for the detector channels. Furthermore, the laser diode operates in a single transverse mode, and the common input/output fiber is a single mode fiber; hence, the path followed by the laser channel must be at all points compatible with single-mode optics, i.e. the laser channel's path must be reversible. In conventional designs, especially those designs using a single diffractive structure in a planar lightwave circuit, there is no practical means of addressing a wide wavelength range (˜1250 nm to 1600 nm) with channels having substantially different passbands.
Prior art devices, such as the triplexer 1 disclosed in U.S. Pat. No. 6,493,121 issued Dec. 10, 2002 to Althaus and illustrated in FIG. 1, achieve the functionality of the triplexer using a number of individually crafted thin film filters (TFF) 2a and 2b, placed in specific locations along a collimated beam path. The TFFs 2a and 2b are coupled with discrete lasers 3 and photo-detectors 4a and 4b, which are packaged in separate transistor-outline (TO) cans 6 and then individually assembled into one component. An incoming signal with the two incoming channels (1490 nm and 1550 nm) enter the triplexer 1 via an optical fiber 7. The first channel is demultiplexed by the first TFF 2a and directed to the first photo-detector 4a, and the second channel is demultiplexed by the second TFF 2b and directed to the second photo-detector 4b. The outgoing channel (1310 nm) is generated in the laser 3 and output the optical fiber 7 via the first and second TFFs 2a and 2b. Unfortunately, the assembly of such a device is extremely labor intensive requiring all of the elements to be aligned with very low tolerances.
Attempts to simplify the housing structure and thereby the assembly process are disclosed in U.S. Pat. No. 6,731,882 issued May 4, 2004 to Althaus et al, and U.S. Pat. No. 6,757,460 issued Jun. 29, 2004 to Melchior et al. Further advancements provided by a triplexer 5, illustrated in FIG. 2, involve mounting the TFFs 2a, 2b and 2c, the laser 3 and the photo-detectors 4a and 4b on a semiconductor microbench 9 ensuring repeatable and precise alignment. Unfortunately, all of these solutions still involve the alignment of TFFs with TO cans. An example of a prior art solution without TFFs is disclosed in U.S. Pat. No. 6,694,102 issued Feb. 17, 2004 to Baumann et al., which discloses a bi-directional multiplexer utilizing a plurality of Mach-Zehnder interferometers.
U.S. Pat. No. 7,068,885 issued Jun. 27, 2006 to the applicants of the present invention discloses a planar lightwave circuit including a pair of face to face diffraction gratings on opposite sides of a slab waveguide, which reflect optical signals off of each other in opposite directions providing diplexer and triplexer functionality.
Concave distributed Bragg reflectors, such as the ones disclosed in U.S. Pat. No. 6,879,441, issued Apr. 12, 2005 in the name of Mosberg, provide simple multiplexer/demultiplexer functionality; however, for the distributed Bragg reflectors, the spectral width of the reflected channel is proportional to the length of the Bragg reflector, e.g. 2 mm of Bragg reflector reflects 10 nm of bandwidth, therefore it would take 20 mm worth of Bragg reflector to reflect a band of 100 nm. Unfortunately, in applications such as diplexers and triplexers that require 100 nm of bandwidth to couple a laser diode channel, the cumulative size of the Bragg reflectors for all three channels would be too large for practical applications.
An object of the present invention is to overcome the shortcomings of the prior art by providing a planar lightwave filter circuit with mixed diffraction elements, whereby the advantages of the different types of diffraction filter elements can be exploited in a single planar lightwave filter circuit.